Years ago, when a solitary lung shadow was found with a known cancer elsewhere, it w a s presumed to be a metastasis and as such, the visible representative of many far-flung metastases not readily observable; hence there seemed to be little point in aggressive action. However, from 1933 through 1972 at Memorial Sloan-Kettering Cancer Center, 626 such solitary lung skadows proved to be separate primary cancers of the lung instead of a metastasis. During the same period, there were 196 solitary pulmonary metastases and 12 benign conditions. The true nature of these equivocal shadows can be determined only by microscopic analysis. It cannot be derived from symptoms, roentgenographic interpretation, the time interval between the known cancer, and the shadow's appearance or the pathological nature of the primary tumor. Therefore, all efforts to obtain histologic material should be made without delay. Multiplicity, of itself, does not portend a poorer prognosis, therefore each cancer should receive definitive and not tentative treatment. Following this precept, there was a 30% 5-year survival after the lung surgery. Diagnostic dilemmas exist when two or more cancers microscopically resemble each other. Several aids to help distinguish a new primary from a solitary metastasis are discussed. Implicit in this study is t4e necessity for cancer patients to have lifetime surveillance at regular intervals, at which time chest roentgenograms should be taken. If a solitary shadow appears, it should be considered a unique and ambiguous situation and aggressive diagnosis and treatment should be instituted without delay.